It is well known in the photographic art that information storage in a photographic film is improved by using an emulsion having both large and small grains of silver halide. The presence of small grains enhances resolution while the presence of large grains enhances speed. A more detailed discussion of the relationship between grain size diversity and information storage is found in a book by C. E. K. Mees and T. H. James, entitled, The Theory of the Photographic Process, 3rd Edition, Macmillan Co. N.Y., 1966.
The applicant has discovered that due to the nature of the latent image development process, large and small grains require disparate development conditions. This discovery is based on applicant's theory which will be described briefly hereinbelow.
The latent image appearing on a silver-halide emulsion is the result of a thermodynamic change in the state of a silver halide grain contained in the emulsion as the result of photon absorption. More specifically stated, the redox potential of the grain E.sub.g changes in such a way that the exposed grains have a greater tendency to accept electrons than the unexposed grains.
The relationship between the redox potential of the grain, E.sub.g and the exposure may be expressed by the following equation: EQU E.sub.g =(R T/F) ln P+constant (1)
where P is the exposure:
R is the gas constant PA0 T is the Absolute Temperature PA0 F is the Faraday constant PA0 T is the duration of the first stage of development PA0 H is a constant that is determined inter alia by the redox potential of the developer E.sub.D PA0 A is the product of the area of the silver halide grains and the the developer concentration; PA0 p is the number of photons absorbed per grain of silver halide and a, b and p' are constants. PA0 It follows from equation (2) that for a fogged (unexposed) grain, e.g. where p=1, T will be large and for an exposed grain where p&gt;&gt;1 T will be small.
The different tendency to accept electrons E.sub.g of different grains is expressed in the latent image during development. If one considers the corrosion model of development described, for example in A. Hoffman, et. al., Photographic Science and Engineering, Vol. 18, 1974, p. 12, there are two different sequential stages of reduction of the silver halide grain by the developer. During the first stage, there occurs a reaction, which is not limited by the surface area of the silver speck, in which developer ions D.sup.- collide with the surface of the grain and whereby electrons are transferred at a generally constant and slow rate from the developer to the grain for reduction thereof.
In the first stage of development, the kinetics are zero'th order, that is, that the rate of change over time of the silver speck resulting from reduction of the silver halide is a constant A proportional to the product of the surface area of the grain and the developer ion concentration.
During the second stage of development an auto-catalytic reaction takes place in which developer ions inject electrons directly into the silver speck. This second stage is obviously dependent in its rate on the surface area of the silver speck.
Equation (1) above may be cast into the corrosion model to generate an equation which defines the duration of the first stage: EQU T=H/A (p'/p).sup.a/b ( 2)
where
The relationship between the redox potential of the developer E.sub.D and the induction period T is: EQU E.sub.D .apprxeq.K.sub.1 log T+Constant (3)
and the relationship between E.sub.D and the developer concentration
(D.sup.-) is: EQU E.sub.D .apprxeq.K.sub.2 log D.sup.- +Constant (4)
where K.sub.1 and K.sub.2 are constants.
It can be generally stated that
T unexposed grain&gt;T&gt;T exposed grain
where T is the residence time of the silver halide grain in the developer.
For a given development process, that is, for a given developer E.sub.D in a given concentration of D.sup.-
1. Small unexposed grains will have a small value of A and thus a relatively large T, for example, 30 seconds. Exposing the small grains will decrease T, for example to a third, or 10 seconds. Thus if the development time T is set at for example 20 seconds, the exposed grains will enter Mode II, (the auto-catalytic development process) and be developed, while the unexposed grains wll not enter Mode II and will remain undeveloped. The result is desired image-wise development.
2. Large unexposed grains will have a large value of A and thus a relatively small T, for example 3 seconds. Exposure of the large grains will decrease T in accordance with equation (1) for example to a third, or one second. In this case there is no practical development time T since a development time of 2 seconds will not allow for complete stage II development of the exposed grains and a development time in excess of 3 seconds permits development of unexposed grains resulting in non-image wise development, which is not desired.
It may thus be appreciated that the development of large grains requires a less potent development process than the development of small grains. The large grain developer must have a lower tendency to give up electrons E.degree..sub.D for increasing H in equation (2) and a lower concentration of D.sup.- for lowering A in equation (2).
Summarizing the above discussion, it may be appreciated that large and small grains require diametrically opposed development conditions. The use of any single development process for both grain sizes involves a compromise which of necessity results in the loss of information contained in the exposed emulsion due to non-image wise development thereof. Furthermore, if the large and small grains are in intimate contact with each other, the oxidation products of the development of the large grains, which according to the above teaching, generally develop first, will interfere with development of the small grains and vice versa.
The present invention seeks to provide photographic material and techniques for increasing the information content and capacity of a photographic emulsion in accordance with the above theory.
There is thus provided in accordance with an embodiment of the present invention a radiation sensitive article formed with a population of large grains and a population of small grains of silver halide arranged for exposure to the radiation and disposed so as to permit development of the large and small grains each under independent development conditions selected to provide image wise development of each thereof giving two coincident or superimposed images.
Further in accordance with an embodiment of the present invention, the large grains may be disposed on one side of a transparent impermeable film and the small grains may be disposed on the other side of the film to permit separate development thereof to provide two superimposed images.
Additionally in accordance with an embodiment of the present invention emulsions on opposite sides of a film and having the same silver halide grain size may be treated under different conditions to produce different silver halide grain sizes.
Further in accordance with an embodiment of the invention there is provided a method for producing a high resolution high speed photographic image comprising the steps of:
preparing radiation sensitive material having relatively large and relatively small grains of silver halide;
arranging the silver halide grains so as to permit the large and small grains to encounter different development conditions;
exposing the radiation sensitive material; and
developing the large grains with a relatively weak developer and developing the small grains with a relatively strong developer, so as to permit image wise development of both the large and small grains.
Further in accordance with an embodiment of the present invention the arranging step may comprise providing separate layers of large grain and small grain emulsions. Alternatively the individual grains may be separately encapsulated with materials which result in different development conditions.